1. Field of the Invention
The present invention relates to a metal material for storing hydrogen stably at a high density.
2. Description of the Prior Art
Ways have been developed for storing hydrogen in metals or alloys in the form of metal hydride. The hydrogen retained in a hydrogen storage metal can be easily stored and transported and the metal can be applied to such purposes as hydrogen separation and purification as well as to heat pumps, etc. Typical metal alloys developed for the storage of hydrogen include TiFe, TiMn, TiCo, LaNi.sub.5, Mg.sub.2 Ni and the like. For instance, U.S. Pat. Nos. 3,508,414 and 3,516,263 disclose a method for storing hydrogen using TiFe alloy, and U.S. Pat. No. 4,079,523 also discloses an iron-titanium-mischmetal alloy for hydrogen storage. Each of these hydrogen storage alloys has its own specific characteristics as regards activation, plateau, hysteresis, etc. and numerous studies have been conducted on how the properties of these alloys can be better adapted to specific applications through the addition of other metal elements.
In a metal material intended for hydrogen storage, the term "plateau" refers generally to the horizontal flatness of an absorption or desorption isotherm obtained from the equilibrium hydrogen pressure-atomic ratio (H/M, where H refers to the atomic weight of hydrogen and M to the atomic weight of a metal element). In case the plateau is not horizontally flat, at the time of hydrogen desorption, the hydrogen pressure is so gradually decreased that hydrogen hardly desorbs unless the desorption pressure tends to be low. Further, at the time of hydrogen absorption, with the increase in the amount of hydrogen storage, hydrogen is hardly absorbed unless the hydrogen pressure is much increased.
In the use of a hydrogen storage alloy having a plateau for storing hydrogen, it is difficult to maintain the hydrogen desorption pressure at a constant value. Further with a decrease of the desorption pressure, the amount of desorption decreases. In the use of the hydrogen storage alloy in an air conditioning system or in a heat engine of the waste heat recovery type, it is desirable that the pressure difference (.DELTA.P) between the equilibrium dissociation pressure of two different types of alloys and the equilibrium absorption pressure of the other alloy should be constant independently of the hydrogen content of the alloy (H/M). When the plateau property is horizontally flat, the hydrogen desorption and absorption reactions of both alloys proceed smoothly since the pressure difference (.DELTA.P) is maintained constant.
In the TiFe alloy, in order to activate the alloys to be reactive to hydrogen, it is required to periodically subject the alloy to elevated temperatures of higher than 400.degree. C.-vacuum and to high pressure hydrogen treatment (30-60 Kg/cm.sup.2) at room temperature for periods as long as one week. It has been found that this disadvantage can be overcome by replacing a part of the iron with Nb, Mn or Ti. However, although the addition of such an element improves the activation, use of Nb greatly increases the cost of the alloy while use of Mn or Ti disadvantageously results in variation of the hydrogen equilibrium dissociation pressure, thus causing the plateau property to become unsatisfactory. In addition, in the TiMn alloy, in order to enhance the plateau property, the addition of other elements such as Zr, V, and Cr has been disclosed.
In general, when the alloy absorbs hydrogen, its volume expands by 10-30%. The repetitive absorption and desorption of hydrogen in the alloy is thus accompanied by repetitive expansion and contraction of the alloy with the result that the alloy is pulverized. Alloys such as FeTi-oxide, FeTiNb, FeTiMn, FeTiNbZr, etc. are known for their easy activation but they tend to be pulverized easily. The pulverized alloy will intermingle into the hydrogen gas desorbed from the alloy to cause the pipes and valves of the system to be clogged. Moreover, when it diffuses into the open air, it gives rise to environmental pollution.
Thus, although methods for improving the characteristic of hydrogen absorbing alloys by the addition of metal elements have been proposed, there still remain unsolved problems in connection with production cost, plateau property, amount of hydrogen storage, service life and various other practical aspects.